1. Field of the Invention
The present invention relates to motion sensors. More particularly, the present invention relates to sensing rotation and angular position in a body, even if the body is not rotating.
2. Background of the Invention
There are many applications where it is desirable to measure absolute angle or inclination. Examples include guidance and navigation systems, construction tools and equipment, and rollover protection devices for vehicles.
Various techniques have been implemented in the past to measure angle, including bubble-based inclinometers (based on the concept of a standard carpenter's level), pendulums, and accelerometers. Each of these technologies is capable of measuring angle. Undesirably, however, these technologies also produce output when subjected to linear acceleration. The automotive rollover application is a particularly good example because deployment of air bags and other safety measures is controlled in part by a rollover indication. It is highly undesirable for the sensor to indicate that the vehicle is rolling over when in fact it is accelerating or going around a corner (producing centrifugal acceleration) as such indication could trigger application of the vehicle's airbags.
Various techniques have been implemented or proposed in the past to avoid the problems due to linear acceleration. Each has its own disadvantage. Mechanical gyroscopes are capable of responding to rotation while rejecting linear acceleration. However, they are subject to drift problems and can be quite cumbersome, expensive, and fragile. Fiber optic gyros solve many of these problems, but respond to angular velocity, rather than absolute angle. Sensors which respond to angular velocity are useful in many applications, but when absolute angle is needed, such as detecting an unstable roll angle in a vehicle, a measure of the absolute angle is desired.
Other techniques such as tuning fork assemblies (see "Detection of Incipient Rollovers Grows in Importance", Automotive Engineering, September 1997, pp. 94-96 ("Automotive Engineering")) and Faraday-effect devices (see U.S. Pat. No. 3,940,983, "Faraday effect fluid flow and direction indicator", to Greene ("Greene")) can be constructed at relatively low cost, but also provide angular velocity information, rather than absolute angle information.
Some conventional devices used multiple fluids in a cylindrical or spherical container (see U.S. Pat. No. 4,779,353, "Tool for measuring inclination and rotation", to Lopes, et al. ("Lopes") and U.S. Pat. No. 5,416,977, "Pitcb Sensor System", to Striffler ("Striffler")). Although theoretically providing absolute angle information, the devices are susceptible to failure (i.e., provide erroneous results) if there is mixing between the fluids or if the boundary layer between the fluids changes. Other devices exist which use fluids that move in response to angular velocity. Each has properties similar to the above described techniques. (see U.S. Pat. No. 4,361,040, "Integrating Angular Accelerometer", Taplin, et al. ("Taplin"), and U.S. Pat. No. 4,163,325 "Verticality Sensors", to Hughes ("Hughes")).
To provide an estimate of absolute angle, hybrid sensor approaches have been proposed and implemented. One common technique used in automotive rollover applications is to combine an accelerometer (or other gravitationally-sensitive device) with an angular rate sensor (see Automotive Engineering). Static angle measurements are made whenever the vehicle appears to not be undergoing acceleration (e.g., when the measured acceleration is 1.0 G, the acceleration due to gravity). Based on this reference static angle, the output from the angular rate sensor is integrated to produce an estimate of absolute angle. Although this approach can be accurate when the vehicle is relatively stable, and when any rotation is high in angular velocity, it is prone to significant errors in integration. The integration problem is more acute where integration needs to be performed for a significant amount of time (a second is often significant). In this case, if there is even a small offset in the output of the angular rate sensor, the integration will have a cumulative error which grows larger with time.
Thus, it would be preferable to have a sensor which provides no cumulative angle measurement error, and which can measure absolute angle regardless of the linear acceleration experienced by the device.